posted on 2024-02-15, 11:33authored byChristopher
S. O’Bryan, Yongliang Ni, Curtis R. Taylor, Thomas E. Angelini, Kyle D. Schulze
Simple
synthetic and natural hydrogels can be formulated to have
elastic moduli that match biological tissues, leading to their widespread
application as model systems for tissue engineering, medical device
development, and drug delivery vehicles. However, two different hydrogels
having the same elastic modulus but differing in microstructure or
nanostructure can exhibit drastically different mechanical responses,
including their poroelasticity, lubricity, and load bearing capabilities.
Here, we investigate the mechanical response of collagen-1 networks
to local and bulk compressive loads. We compare these results to the
behavior of polyacrylamide, a fundamentally different class of hydrogel
network consisting of flexible polymer chains. We find that the high
bending rigidity of collagen fibers, which suppresses entropic bending
fluctuations and osmotic pressure, facilitates the bulk compression
of collagen networks under infinitesimal applied stress. These results
are fundamentally different from the behavior of flexible polymer
networks in which the entropic thermal fluctuations of the polymer
chains result in an osmotic pressure that must first be overcome before
bulk compression can occur. Furthermore, we observe minimal transverse
strain during the axial loading of collagen networks, a behavior reminiscent
of open-celled cellular solids. Inspired by these results, we applied
mechanical models of cellular solids to predict the elastic moduli
of the collagen networks and found agreement with the moduli values
measured through contact indentation. Collectively, these results
suggest that unlike flexible polymer networks that are often considered
incompressible, collagen hydrogels behave like rigid porous solids
that volumetrically compress and expel water rather than spreading
laterally under applied normal loads.